Why is the Affordable Care Act necessary?

The Patient Protection and Affordable Care Act, popularly known as "Obamacare," is designed to provide affordable healthcare to individuals and families. The legislation, which was signed into law in 2010, does three very important things: it has maintained consumer choice in healthcare while offering affordable plans based on household income, it allows both married and unmarried young people to remain on their parents' insurance plans up to the age of 26, and it has eliminated pre-existing conditions as a basis for insurance companies to refuse people health insurance coverage. These clauses in the ACA have expanded insurance coverage for millions of Americans who were previously uninsured.
Though Republicans strongly opposed the ACA upon its introduction in 2009, the idea for marketplace insurance emerged out of the conservative think-tank, The Heritage Foundation. Also, the Republican Mitt Romney installed a very similar program when he was governor of Massachusetts, though he spoke out against Obamacare on the campaign trail, due to the legislation's unpopularity among conservatives.
Currently, Republicans are working, probably due to encouragement from insurance companies, to eliminate the clause about ensuring coverage for those with pre-existing conditions. When uninsured people get sick or injured, they must be treated. The cost of treating the uninsured raises healthcare costs which, in turn, raises insurance premiums. Therefore, it is better for as many people to be insured as possible. Those who remain uninsured are subjected to an additional tax when the time comes to file income tax.
The complaint among some people is that Obamacare is still unaffordable and that they were better off with their previous insurance plans. Firstly, just because a plan is affordable does not mean that it provides sufficient coverage. HMOs (Health Management Organizations) have provided coverage to customers for an annual fee. However, many HMOs are notorious for providing insufficient coverage. Secondly, the high costs of health insurance are linked to the high costs of healthcare in the United States, which are much higher than those in other developed nations. There is no cap on healthcare costs in the United States, meaning that hospitals, doctors, and other care providers can charge what they please for supplies and services. The Affordable Care Act does not address exorbitant healthcare costs, which are continually on the rise.
https://www.hhs.gov/healthcare/about-the-aca/index.html